1. Field of the Invention
The present invention relates to a process for producing an olefin polymer. More specifically, it relates to a process for producing an olefin polymer by polymerizing an olefin or olefins by a multi-stage polymerization in the presence of a catalyst.
2. Description of the Related Art
A process for polymerizing an olefin by a plurality of divided stages is known in the art. One object of the polymerization by a plurality of stages is to broaden the molecular weight distribution or composition distribution of an olefin polymer. In this process, polymers having a different molecular weight or composition ratio are prepared in the respective stages, and then kneaded with each other to prepare a polymer having desired properties. Another object of the polymerization by a plurality of stages is to produce a "propylene-ethylene block copolymer". In general, a homopolymer of a propylene or a propylene copolymer containing a minor amount of another monomer is prepared in the former part, a propylene-ethylene copolymer having a relatively low propylene content and a very highly amorphous polymer component content is prepared in the latter part, and the polymers thus prepared are kneaded with each other to prepare a propylene-ethylene block copolymer having a well balanced impact resistance and rigidity.
The above-described multi-stage polymerization often causes a problem in that the appearance, impact resistance and other mechanical properties of a molded article of a final polymer become poor due to an unsatisfactory compatibility of the polymer prepared in the former part with the polymer prepared in the latter part. The primary cause of the deterioration of the appearance and mechanical properties is considered to be as follows. Specifically, when the polymerization is conducted continuously by using a plurality of reaction vessels connected in series, a distribution of the residence time of the catalyst occurs in each reaction vessel, which often causes catalyst particles having substantially no residence time in the former part to enter the latter part (such a catalyst particle is hereinafter referred to as a "short-cut particle"). Polymer particles prepared in the latter part in the presence of catalyst particles having a residence time in the former part and coated with the polymer prepared in the former part are in a micro state, i.e., the polymer prepared in the former part and the polymer prepared in the latter part are dispersed in a micro state, and thus the above-described problem does not occur. On the other hand, only a polymer in the latter part is prepared from the short-cut particles, which causes a poor dispersion, and thus the above-described problem arises.
The following methods and attempts have been proposed for alleviating the above-described problem.
1) A method wherein a plurality of reaction vessels are provided in the former part of the polymerization and are switched from one to the other at predetermined intervals, to thereby conduct the polymerization substantially in a batch-like manner. PA0 2) An attempt to reduce the amount of "short-cut particles" as a whole through the use of a plurality of stages in series in a portion corresponding to the former part of the polymerization. PA0 3) An attempt to reduce the amount of production of a polymer having a poor dispersibility, through the addition of a reagent for controlling the polymerization activity of the "short-cut particles" in the latter part of the polymerization (see U.S. Pat. Nos. 4,492,787 and 4,739,015, and Japanese Unexamined Patent Publication Nos. 55-115417, 57-174311, 61-69823, 63-41518, 63-46211, 63-75005 and 63-75009). PA0 (a) a polyfunctional electron donor compound and PA0 (b) a compound different from the compound (a) and selected from a compound having an M-O-R group, oxygen, carbon monoxide and carbon dioxide is added at the second polymerization stage.
Among the above-described methods and attempts, the method and attempt described in the above items 1) and 2) require an increase in the amount of equipment needed, whereby the production cost of the resultant polymer is increased, which renders the method and attempt described in the above items 1) and 2) disadvantageous from the viewpoint of profitability.
On the other hand, according to studies made by the present inventors, the attempt described in the above item 3) is useful for a particular catalyst used in the olefin polymerization but has a poor generality or still has an unsatisfactory effect. In particular, the application of this attempt to Ziegler catalysts, especially catalysts for stereospecific polymerization used in the polymerization of, for example, propylene and copolymers having a high adherence, is unsatisfactory, from a practical viewpoint because the effect is poor.